The present invention relates to a contact sensing head and is particularly, but not exclusively, related to a sensing head for a measuring device operable to detect the position of the points in space with regard to which mechanical contact takes place between a sensor of the sensing head and an object to be measured.
Such measuring devices may be operated by securing the object on a mobile workpiece-carrying table and by moving the points to be measured into the region of the sensing head, which is capable of locating them with a desired degree of precision relative to a reference origin. When the positions of the sensing head itself and of the table have been determined by the measuring system of the machine relative to a common reference origin and according to selected coordinate axes, it is an easy matter to deduce therefrom, by calculation, the positions of the aforementioned points measured relative to the said origin.
There are measuring heads which supply an "all or nothing" electrical signal as soon as mechanical contact is established, at the point to be measured, between the sensor and the object. In this case, in order to ascertain the position of the point of contact, the procedure is to utilise the said electrical signal for "recording in flight" or "fixing" the coordinates indicated by the measuring system of the machine at the instant of contact, and to add to the said coordinates the constants due to the geometrical form of the sensor.
In known embodiments of such measuring heads, generation of the electrical signal takes place by employing the following processes:
The mechanical contact establishes an electrical current between the sensor and the object;
Electrical contact or contacts establishing the signal are actuated by the sensor; they are frequently mounted on guides having a zero degree of freedom and maintained by springs (see German Patent Specification No. 25 51 361 and British patent specification No. 1,447,613);
The sensor actuates an element the position of which is detected by magnetic, capacitive or photoelectrical means producing an analog voltage approximately proportional to the displacement of the said element (German Patent Specification No. 26 11 781);
The useful signal of the "all or nothing" type is produced by a voltage comparator during passage to zero of the said analog voltage;
A strain gauge system of the resistance wire or semiconductor type is fixed on the sensor itself or on a support thereof, constituted for example by flexible blades; it converts the deformation of the sensor or of its support on contact with the object to a proportional analog voltage (French Patent Specification No. 73 23 193, publication No. 2 235 351); as in the preceding case, the passage to zero of the said voltage is located by a voltage comparator which produces an all or nothing signal; these systems generally have the following defects:
1. Non-constant quality of electrical contact
2. Vibration of the electrical contact
3. Lack of rigidity, fragility
4. Precision of measurement which is a function of the length of the sensor.
British patent specification No. 932069 relates to a sensor constituted by a piezoelectrical crystal vibrating at its resonant frequency and maintained at this frequency by an oscillating circuit. Application of the said sensor is limited to that of signalling the advance of the cutting tool of a lathe. The said sensor reveals the instant of passage of the tool, coming always from the same direction. Such an embodiment cannot be envisaged for universal application as in a measuring head, in which case the direction in which measurement is effected is optional but precision of measurement must remain non-variable. Furthermore, in the British patent specification No. 932069, the sensor itself is constituted by the piezoelectric crystal and this construction makes it extremely fragile and vulnerable to untimely shocks.